1. Field of the Invention
This invention relates to a device for monitoring the time-temperature history of an article in which a vapor is allowed to permeate through a porous substrate coated with an indicating solid, which undergoes a color change upon contact with the vapor, thus forming a moving boundary as vapor permeates through the substrate.
2. Brief Description of the Prior Art
A host of perishable products such as frozen foods, blood, vaccines and the like require a means for monitoring time-temperature shelf-life histories. Various indicators have been described in the prior art as meeting this need with varying success.
Indicators are known that use a liquid or liquid vapor for activating a color response when a predetermined shelf life has expired. For example, U.S. Pat. No. 3,844,718 discloses a defrost indicator which is activated by the contact of water or water vapor with a water-soluble ink supported on a hygroscopic substrate.
U.S. Pat. No. 3,768,976 discloses a time-temperature indicator that depends upon the rate of permeation of oxygen through a polymer envelope containing an aqueous solution of a red redox dye. Upon oxidation, the red dye turns colorless, indicating that the perishable has been exposed to too high a temperature for too long a time.
U.S. Pat. No. 3,915,719 discloses a temperature indicator in which a frozen liquid is separated from an indicating layer by a liquid-soluble barrier. When the frozen liquid thaws, a time delay is introduced by the rate of dissolution of the liquid soluble barrier. Only upon dissolution of this barrier does color indication occur.
There is a continuing need for improved indicators for monitoring the time-temperature histories of perishable products. Known successful devices require either liquids or gases. However, a device requiring vapor for initiation of the color response would be relatively less expensive to produce since less liquid would be required than in a device requiring liquid activation. Also, such a device would be without the disadvantage of depending upon the diffusion of an external gaseous reactant such as oxygen. Such as dependence can result in unwanted variations in device response for different environments. Moreover, the prior art does not suggest such that a device could be effectively used, in which device operation is based solely on the interaction between a vapor and an indicating solid capable of undergoing a color change, upon contact with the vapor.